Allow to define the namespace/package separator

PHP as example uses a back slash as its namespace separators. But in the current sources the dot as package separator is hard coded to generate the client classes.

It would be nice if the generator would introduce a property which defines this separator, so that it can be overwritten by the language specific generators.

This pull request adds a `packageSeparator` property in the `CodegenConfig` which defaults to the dot as package separator. It adds also an helper method `toPathName` which converts a package name into a path name. I have replaced all these conversions in the sample codgen files with the new helper method.

Furthermore I've added new samples called `php-modern` to demonstrate the creation of PHP files with namespaces. Please let me know if you have a more suitable name for the samples.